A synchronous machine includes a field coil-type synchronous machine, a permanent magnet-type synchronous machine and a reluctance motor. In the field coil-type synchronous machine, field magnetic flux of a rotor is generated by an excitation coil (field coil). In the permanent magnet-type synchronous machine, magnet flux generated by permanent magnets is used as filed magnetic flux. In the reluctance motor, reluctance torque is generated by magnetic resistance variation of a salient pole-type rotor core, which rotates relative to a salient pole-type stator core.
The permanent magnet-type synchronous machine is widely used for high efficiency use, because motor efficiency can be raised owing to no excitation loss in generating field magnetic flux. In the permanent magnet-type synchronous machine, a stator coil generates counter-electromotive force in a high speed rotation range, if it is operated over a wide rotor rotation speed range. It is the most common excitation reduction measure to configure a rotor to generate magnet magnetic flux torque and reluctance torque and reduce the magnet magnetic flux by d-axis current magnetic flux by adjusting d-axis current. However, if the d-axis current is reduced, loss due to the d-axis current is large.
In addition, a hybrid excitation system is proposed (JP 2006-141106A). In this system, a magnetic path (magnetic path of excitation coil), about which an excitation coil is wound, is provided in parallel to a magnetic path (magnetic path of a permanent magnet) having a permanent magnet in a magnetic path of a magnetic circuit (magnetic circuit of field magnetic flux), in which magnetic flux flows. By adjusting a current (excitation current) of an excitation coil, the amount of field magnetic flux in an electromagnetic gap between a rotor and a stator in the magnetic circuit of the field magnetic flux is adjusted. This synchronous machine, which uses both permanent magnets and an excitation coil and is operated by a hybrid magnetization system, is referred to as a hybrid excitation-type synchronous machine. In the hybrid excitation system according to JP 2006-141106A, the magnetic path of the excitation coil and the magnetic path of the magnet are arranged in parallel. For this reason, this magnetization system is referred to as a parallel-type hybrid excitation system.
However, according to the parallel-type hybrid excitation system, the magnetic flux of the magnet bypasses the magnetic path of the excitation coil if the excitation is not increased or decreased. As a result, the amount of magnetic flux of the magnet that crosses the stator coil decreases and hence torque decreases. If a large amount of field magnetic flux is required, an excitation current need be supplied to the excitation coil to generate a relatively strong magnetic filed. As a result, excitation loss increases.
The magnetic path of the excitation coil according to the conventional parallel-type hybrid excitation system is normally formed by a motor frame, which supplies and receives magnetic flux to and from the rotor core through the stator core. As a result, it is required to form a large motor frame by the use of soft steel, which is far heavier than aluminum, for example. Thus, the weight of the motor increases, and a part of the magnetic flux of the excitation coil is likely to leak to a bearing section.